In the United states, the short answer is any signature, whether made with a pen or a finger can be contested by the person who signed them. Digital document are no more likely to be contested than any other form of signature.
The in-depth answer is anything can be contested as legally binding but all documents can be entered as evidence during the discovery phase of a lawsuit. If the case goes to trial, documents can be contested as admissible evidence. At that point, the documents themselves will most likely be contested on the grounds of some aspect which isn't the way it was signed.
Electronic signatures are legally binding in many countries. In the UK for example, under the Electronic Communications Act 2000, signing a contract using an electronic signature is legally binding in the same way as signing a paper version of a contract. For the purposes of legal proceedings in the UK, electronic signatures are admissible in evidence in relation to the authenticity of the contract. This includes proceedings in all parts of the UK including Northern Ireland. If you intend to use an App for contracts that are not subject to the laws and jurisdiction of the UK, keep in mind we're nerds, not attorneys. It is your responsibility to obtain legal advice (prior to using ‘sign’ functionality) to ascertain whether electronic signatures are valid under the laws and jurisdiction applicable to your contract.